<p>The rock-socketed pile is a type of deep foundation where the pile tip is embedded into bedrock to enhance bearing capacity and stability. The shear behavior of the pile–rock interface is a critical factor governing the load-transfer mechanism and ultimate bearing capacity of rock-socketed piles. Conventional methods for assessing interface roughness are often oversimplified, leading to significant deviations in predicting interface shear strength under high stress conditions. This paper proposes a comprehensive quantification method for pile–rock interface roughness based on integrating three key morphological parameters: fractal dimension, inclination angle, and asperity height. A series of direct shear tests were conducted on rock–concrete interfaces under varying normal stresses and surface roughness conditions. The results demonstrate that both the peak and residual interface shear strengths increase significantly with increasing roughness and normal stress. Furthermore, addressing the issue where the Barton–Bandis empirical model may yield a peak friction angle approaching 90° under high roughness and stress conditions, leading to distorted predictions of ultimate shear strength, an improved model is presented. This model incorporates a cohesion term and a shear-off failure weighting factor. Finally, the proposed model was validated based on pull-out tests of large-diameter rock-socketed piles by geotechnical centrifuge model test, from which failure modes, load–displacement curves, axial force transfer, and side friction resistance were obtained. This study provides a mechanism-driven, parameter-measurable, and engineering-test-verifiable new approach for the strength characterization of complex rough interfaces and the assessment of pull-out capacity.</p>